The present invention is directed to systems having a plurality of power supplies. A preferred embodiment of the present invention is directed to systems that have a battery power supply and an alternating current power supply.
Two problems may occur when switching power supplies such as, by way of example and not by way of limitation, when switching between battery power supply and a dc power supply such as would be derived from an AC adapter for a notebook computer or a handheld computing or communication device. First, when powering up the system using an AC adapter power supply, high peak currents (e.g., in excess of 70 Amps) may occur. Such a condition may cause reliability problems and usually adds to the cost of the system because of increased robustness of components required to withstand the rigors of high inrush current such as, by way of example and not by way of limitation, increased robustness of protection components and switches. Second, there can be reliability issues relating to switching between power supplies that have voltages higher than the ratings for commercially available PMOS (P-channel Metal Oxide Semiconductor) power switches (i.e., approximately 20 volts). Circuit designers commonly have added protection circuits to overcome this reliability issue. However, the protection circuitry may sometimes effect a lock up condition when switching between power supplies at high voltage.
There is a need for an apparatus and method that can effect switching between power supplies that limits current inrush when powering up a system using an AC adapter-derived power supply.
There is a need for an apparatus and method that can effect switching between power supplies that avoids a lock up condition when switching between power supplies.